Telephonic communications are increasingly being conducted through wireless radio frequency (RF) broadcast rather than wire conduction. For example, both cordless and cellular telephones communicate with stationary transmitters via RF broadcast signals.
A wireless telephone handset contains an RF transmitter and receiver, and an RF antenna. The antenna receives an RF signal and conducts it to the receiver, which amplifies and demodulates it to produce an audio output signal for the handset's earpiece. To optimize the amplification of the signal, the amplifier's input impedance must be matched with that of the antenna or front-end RF filter (typically a resistance of 50 ohms). Input impedance-matching prevents signal reflection and delivers maximum input signal power to the amplifier. Preferably, such impedance-matching should be accomplished without introducing noise into the RF signal from the antenna.
Several methods of matching the input impedance of an amplifier to an antenna impedance have been devised. One such method is to use a shunt feedback resistor for negative feedback. Suppose an inverting amplifier has a high input impedance and produces a signal gain -A. If a feedback resistor with resistance R.sub.F is connected from the amplifier's output to its input, then the input impedance of the amplifier circuit will become approximately ##EQU2##
By choosing a proper value of R.sub.F, the impedance of the antenna can be matched across a wide band of signal frequencies. However, the feedback resistor introduces an undesirable level of thermal noise into the RF signal.
Another method of impedance-matching is to connect a network of lossless passive elements such as inductors and capacitors (a "matching network") to the amplifier input stage. This allows impedance-matching across a selected narrow band of signal frequencies, which is sufficient for most telecommunications applications. However, since high quality inductors cannot easily be formed on a silicon integrated circuit chip, such a matching network cannot be formed on the same integrated circuit chip as the amplifier and other signal processing electronics. This is due to the thermal loss (heat generation) associated with on-chip inductors, which adds to the noise of the receiver. This creates a necessity for external or off-chip circuit components, which increases both the size and the cost of the signal processing system.
Thus, it is desirable to have a method of matching the input impedance of an amplifier with the resistive impedance of an antenna or RF filter using an integrated circuit component. It is also desirable to accomplish the impedance-matching without introducing noise into the amplified signal.